What is the primary action of propylthiouracil (PTU) in the treatment of hyperthyroidism?

Propylthiouracil (PTU) primarily acts by inhibiting the synthesis of thyroid hormones, specifically triiodothyronine (T3) and thyroxine (T4), produced by the thyroid gland. It achieves this by blocking the enzyme thyroid peroxidase, which plays a crucial role in the iodination of tyrosine residues in thyroglobulin and the coupling of iodotyrosines to form T3 and T4. By reducing the production of these hormones, PTU effectively decreases the levels of circulating thyroid hormones, which is essential in managing hyperthyroidism, a condition characterized by excessive hormone production.

The other options focus on the regulation of thyroid-stimulating hormone (TSH) or the production of thyroid hormones in ways that do not accurately represent the mechanism of action of PTU. For instance, PTU does not significantly affect the levels of TSH directly; rather, it influences the feedback mechanism, which may indirectly impact TSH levels over time but is not its primary action. Therefore, the focus on the direct inhibition of T3 and T4 synthesis highlights why the correct answer identifies PTU's primary function in treating hyperthyroidism.